1. Field of the Invention
The present invention relates to a numerically controlled machine tool capable of machining a non-circular workpiece such as a cam shaft.
2. Description of the Prior Art
Generally, in a numerically controlled grinding machine, the feed of the grinding wheel perpendicular to a spindle axis is controlled by a numerical control apparatus according to profile data and machining cycle data.
The profile data comprises the amount of movement of the grinding wheel per unit angle rotation of the spindle which defines the reciprocating motion of the grinding wheel along the finished shape of the workpiece. On the other hand, the machining cycle data comprises a numerical control program to control a machining cycle which includes rapid feed, cutting feed and retracting feed of the grinding wheel.
In the numerically controlled grinding machine tool of the aforementioned type, the ability of the main spindle and the grinding wheel feed axis to accurately follow command data are needed so as to grind the workpiece precisely.
Although increasing the gains of the servo amplifiers is a good method to reduce errors due to following delays, such causes instability in servo control.
On the other hand, a method of compensating the profile data is used to reduce the errors. In this method, a workpiece is ground depending upon ideal profile data which is calculated from the ideal final shape of the workpiece. The final profile of the ground workpiece contains a radial profile error and phase error. After grinding, an operator measures the radial profile error and the phase error with a cam profile measuring device. Then, the operator calculates the execution profile data by compensating the ideal profile data for the radial profile error. At the beginning of grinding the workpiece, the operator shifts the initial angle position of the spindle according to the measured phase error. The amount of follow delay is changed depending on the profiles of the workpieces and the temperature of the servo mechanism, and is changed with the lapse of time. Accordingly, to improve profile accuracy, compensation should be accomplished frequently. Since the above-mentioned measurement and compensation are done manually by an operator, the work efficiency of the operator is poor.
Furthermore, the follow delay of the main spindle is changed depending on the rotational speed of the main spindle. Accordingly, if the speed of the main spindle is changed according to he machining speed, it is difficult to compensate the phase error correctly. Moreover, the follow delay due to the velocity component of profile cannot be compensated completely by the above-mentioned compensation.